I have used bridge domains for this purpose.
In the bridge domain you add the sub-interfaces for all the sites you want to be in the same broadcast domain for the inbound trunk port. It does not matter what vlan tag they have on this interface.
In the bridge domain you assign the vlan tag for your gateway for this broadcast domain.
You can then either send that vlan out layer two on another interface towards the gateway. Or create the IRB layer 3 interface on the MX to serve as the gateway. If you add the gateway on the MX make sure you consider the routing domain you want the traffic to be in. You might also want to create a virtual router routing instance to isolate that traffic.